In recent years, a liquid crystal display device has been prevailing in place of a CRT (Cathode Ray Tube). The liquid crystal display device is widely used as, for example, a liquid crystal television, a monitor, and a mobile phone, by taking advantages such as an energy saving characteristic, a small thickness, and a light weight. Improvement of an illumination device (so-called backlight) that is provided on a backside of the liquid crystal display device is one method for taking such advantages more effectively.
Illumination devices are broadly classified mainly into a sidelight type (an edge-light type) and a direct type. A sidelight type illumination device is provided with a light guide on a backside of a liquid crystal display panel. Side end section(s) of the sidelight type illumination device is(are) provided with a light source(s). Lights that are emitted from the light sources are reflected by the light guide, so that the liquid crystal display panel is uniformly and indirectly illuminated. An illumination device that is realized according to the arrangement can have a reduced thickness and excellent luminance uniformity, though the luminance of the illumination device becomes low. Accordingly, the sidelight type illumination device is mainly used in medium-size and small-size liquid crystal display devices such as mobile phones and laptop computers.
A direct-type illumination device is provided with a plurality of light sources on a backside of a liquid crystal display panel. In the direct-type illumination device, a liquid crystal display panel is directly illuminated. Accordingly, a high luminance can be easily obtained, even in the case of a wide screen. Therefore, the direct-type illumination device is mainly used in a large-screen liquid crystal display device having a screen larger than a 20-inch screen. However, currently, the direct-type illumination device has a thickness of approximately 20 mm to 40 mm. This thickness is an obstacle for further reducing a thickness of the display device.
For further reducing a thickness of the large-screen liquid crystal display device, a distance between light sources and a liquid crystal panel should be shortened. However, in such a case, unless the number of the light sources is increased, luminance uniformity of the illumination device cannot be obtained. On the other hand, in a case where the number of the light sources is increased, cost of the display device increases. Therefore, it is desired to develop, by causing no increase in the number of light sources, an illumination device that has excellent luminance uniformity.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 117023/2005 (Tokukai 2005-117023) (published on Apr. 28, 2005)
[Patent Document 2]
Japanese Unexamined Patent Publication No. 284283/2005 (Tokukai 2005-284283) (published on Oct. 13, 2005)
[Patent Document 3]
Japanese Unexamined Patent Publication No. 307526/2001 (Tokukai 2001-307526) (published on Nov. 2, 2001)
[Patent Document 4]
Japanese Unexamined Patent Publication No. 94008/1995 (Tokukaihei 7-94008) (published on Apr. 7, 1995)